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How to Rebuild the Mississippi Delta

By Kelly Slivka July 25, 2012 1:30 pmJuly 25, 2012 1:30 pm

Jeffrey A. NittrouerA field assistant in an exposed dune field in the Bonnet Carré spillway in Louisiana last July. The sand, left from a diversion of the Mississippi River, extends nearly as far as a low building just below the line of refinery stacks in the background.

You may remember that during last year’s epic Mississippi River flooding, the Army Corps of Engineers opened spillways upriver of New Orleans and Baton Rouge to divert some water away from the cities. One of those spillways was the Bonnet Carré, built in the early 1930’s to protect New Orleans from high water.

When most of its bays were opened last May, the Bonnet Carré redirected 10 to 20 percent of the Mississippi’s floodwater out of the river and into Lake Pontchartrain.

While the diversion mitigated the havoc created by the flood, it also provided a few scientists with an opportunity to do some research on land restoration in the Mississippi Delta as the Bonnet Carré’s floodwater dried out in subsequent months and left behind huge dunes of sand. Their findings, published this week in a Nature Geoscience letter, indicate that well-placed floodwater diversions can add significant amounts of land to the disappearing delta.

Our study “demonstrates that there’s a strong feasibility or potential to build new landscape in Louisiana,” said Jeffrey A. Nittrouer, a geologist at the University of Illinois and the lead author of the letter. He said the recent use of the Bonnet Carré spillway showed that by choosing the right place to build a diversion in the Mississippi and opening it at the right time, planners could build up a substantial amount of sediment in the delta.
Much of the Mississippi Delta is sinking: sands settle lower as they compact over time and as the earth’s crust warps. At the same time, the wetlands are being washed away by the sea – a point underlined when Hurricane Katrina demolished much of Louisiana’s marshland in 2005. Sediments that once washed over the delta during floods, naturally restoring wetlands, are now often blocked by dams and levees and other human activity on the river.

Scientists have been working to find the best way to restore the wetlands, or at least to combat their disappearance, but the outlook has generally been grim. A study published in 2009 in Nature Geoscience concluded that the Mississippi did not have enough sand to combat land loss, regardless of human impact on the river system.

“If you look at the budget of sediments versus the amount of space being created by subsidence and increased sea level rise, there’s not enough sediment in the system to fill that space,” said Harry Roberts, a geologist at Louisiana State University and one of the two authors of the 2009 study.

But Dr. Roberts suggests that all is not lost for the restoration of the delta. “You can’t save it all,” but the recent study points to “encouraging” ways to save some of it, he said.

In the study, the researchers looked at the shape and composition of the Mississippi as it leads up to the spillway – its substrate and depth, the curvature of its bends and the sheerness of its walls and banks. They also compared the volume of sand left behind after the Bonnet Carré spillway dried out to the amount of floodwater that passed through during the diversion.

The results indicate that by putting a diversion in a place with similar mechanics to those of the Bonnet Carré and only opening it during floods – “when the river gets very energetic” – a lot of land can be created in the delta, Dr. Nittrouer said.

“As the results came through, we began realizing just how efficient this particular structure is for building sediment,” he said. While the Bonnet Carré spillway skimmed off only the top 10 to 15 percent of the water column moving down the Mississippi, Dr. Nittrouer and his colleagues estimate that 31 to 46 percent of the sand in the river ended up in the spillway.

Dr. Roberts, who has been conducting research in the area for decades, said that people invested in the delta could harness research like Dr. Nittrouer’s to find strategies for shoring up land. In the course of his career, he noted, Mississippi mud has been transformed from a headache into a prized resource.

“We’re struggling as researchers and managers of the coast with where to apply that important natural resource,” he said. “Picking the right position to get the most out of every diversion that we establish is really important.”

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